Implants, such as dental implants, are well known in the art. They generally consist of a material, which is biocompatible and which additionally has a low elastic modulus and a high strength.
Apart from its biocompatibility and its mechanical properties, the osteointegrative properties of an implant are usually of major importance. The term osteointegration designates the direct structural and functional connection between living bone and the surface of the load-bearing implant. A good osteointegration means that the implant, after reaching a primary stability by screwing it into the bone, safely ossifies within a short healing time so that a permanent bond between implant and bone is obtained.
Dental implants which are currently in use are in general made of a metal, e.g. titanium, or a ceramic, e.g. a zirconia based ceramic, due to the biocompatibility and the favourable mechanical properties of these materials.
In contrast to titanium implants, which are dark and therefore mismatch with the color of natural teeth, ceramic materials have the advantage that their color can be closely matched to the natural tooth color. Efforts have thus been made to provide dental implants, of which at least the parts that are visible after insertion are made of a ceramic material.
Despite their favourable properties with regard to the color, the use of ceramic materials for dental implants is in many cases limited by their fatigue stability, which is generally rather low.
A ceramic material having a high mechanical strength is disclosed in U.S. Pat. No. 6,165,925, which relates to an yttrium-stabilized zirconium oxide in predominantly tetragonal form for the production of a sintered semi-finished article.
In order to achieve a sufficient mechanical stability, the zirconia ceramic disclosed in U.S. Pat. No. 6,165,925 must be highly dense. The surface of such a highly dense zirconia ceramic is clean cut, extremely hard and has essentially no porosity. A dental implant made of such a zirconia ceramic is thus bio-inert and has only weak osteointegrative properties.
For providing an osteointegrative ceramic surface, a variety of different techniques have been suggested:
WO-A-2005/027771 relates to a process for preparing a dental installation in which a dispersion is applied on a substrate having a first porosity, said dispersion forming upon sintering a ceramic layer with a second porosity.
EP-A-0870478 relates to a dental retention element having a core of a high-strength material such as zirconia, said core being coated with a ceramic material which can be chemically and/or mechanically processed.
The composite structures disclosed in WO-A-2005/027771 and EP-A-0870478 have the disadvantage that the ceramic coating is easily chipped off.
Alternatively, a treatment comprising abrasive blasting and acid-etching for providing osteointegrative properties to the ceramic implant's surface has been suggested by EP-B-1450722 and EP-A-1982670.
EP-B-1450722 relates to a dental implant made of zirconia ceramic which after abrasive blasting is subjected to a treatment using phosphoric acid, sulphuric acid, hydrochloric acid or mixtures thereof.
EP-A-1982670 relates to a process wherein a roughness is provided to the surface of the dental implant by sandblasting, milling and/or injection molding techniques prior to the etching of the implant with an etching solution comprising hydrofluoric acid.
Although the process disclosed in EP-A-1982670 leads to an implant having excellent osteointegrative properties, the techniques applied prior to the etching have several disadvantages.
The sandblasting, for example, goes along with a loss of material due to the fact that it is an erosive technique. In particular with regard to ceramic implants, the relatively harsh treatment of the surface might result in the formation of defects and a phase transformation of the tetragonal to the monoclinic zirconia phase and thus in a negative impact on the mechanical stability of the implant.
Providing a structure by milling or injection molding techniques, which are mentioned in EP-A-1982670 as alternatives to the sandblasting, is relatively complicated and thus expensive. In particular, the injection molding techniques require sophisticated approaches for example with regard to the removal from the mold.
A further alternative technique for providing a structure to the surface of a ceramic implant is described in DE-A-102006062712, which relates to a process in which the surface roughness is increased prior to the sintering by applying sharp-edged particles to the surface of the so-called green body and/or brown body. It is thereby preferred that the green body and/or the brown body is coated with a binder for fixation of the particles.
According to another embodiment of the technique described in DE-A-102006062712, the particles are mixed with the binder and applied on the green body and/or the brown body.
DE-A-102006062712 thus teaches single particles to be applied on the surface of the implant body, which leads to protrusions in the shape of the particles. As mentioned above, it is according to DE-A-1020060621712 essential that these particles are sharped-edged.
In order to prevent the implant to be damaged, DE-A-1020060621712 teaches that the particles are “trickled” on the green body and/or the brown body, respectively, without exerting pressure (“drucklos”). As illustratively shown in DE-A-1020060621712, a body with an interface between a basic body and the applied particles is thereby obtained.
The process according to DE-A-1020060621712 is however relatively complicated to perform. In particular with regard to the particles being trickled without exerting pressure, they tend to fall off the surface on which they are applied. Also, due to the fact that the trickled particles tend to accumulate in the valleys of the surface topography rather than on the peaks, a homogenous distribution of the particles is difficult to obtain.